Abstract [en]

Bacterial genome architectures evolve in response to selective pressures on the interplay between replication and gene expression. Several genomes contain a higher fraction of genes coding for proteins involved in information processes near the origin of replication, which is thought to be due to selection for rapid growth. We recently described a novel type of genome architecture in Lactobacillus kunkeei (Tamarit, et al. 2015). In this genome, vertically inherited genes encoding proteins with roles in translation and replication have accumulated in the chromosomal half surrounding the terminus of replication, while species-specific genes, and genes encoding proteins with metabolic and transport functions have accumulated in the chromosomal half around the origin of replication. Here, we show that this pattern is present also in the closest relatives of L. kunkeei, and similar but not identical biased genome architectures are present in other groups within the Lactobacillaceae. Thus, the biased genome structure in L. kunkeei has emerged from an ancestral clustering of vertically inherited genes around the terminus of replication, while horizontally acquired genes have been inserted near the origin of replication. The genome bias has been lost independently in several groups due to insertions of mobile elements near the terminus of replication and/or major genome rearrangements. We propose chromosomal structuring in macrodomains in the Lactobacillaceae, and suggest that further exploration of its functional consequences and generality will provide valuable insights into the forces that shape genome organization in bacteria.

Abstract [en]

This thesis focuses on the genomic study of symbionts of two different groups of hymenopterans: bees and ants. Both groups of insects have major ecological impact, and investigating their microbiomes increases our understanding of their health, diversity and evolution.

The study of the bee gut microbiome, including members of Lactobacillus and Bifidobacterium, revealed genomic processes related to the adaptation to the gut environment, such as the expansion of genes for carbohydrate metabolism and the acquisition of genes for interaction with the host. A broader genomic study of these genera demonstrated that some lineages evolve under strong and opposite substitution biases, leading to extreme GC content values. A comparison of codon usage patterns in these groups revealed ongoing shifts of optimal codons.

In a separate study we analysed the genomes of several strains of Lactobacillus kunkeei, which inhabits the honey stomach of bees but is not found in their gut. We observed signatures of genome reduction and suggested candidate genes for host-interaction processes. We discovered a novel type of genome architecture where genes for metabolic functions are located in one half of the genome, whereas genes for information processes are located in the other half. This genome organization was also found in other Lactobacillus species, indicating that it was an ancestral feature that has since been retained. We suggest mechanisms and selective forces that may cause the observed organization, and describe processes leading to its loss in several lineages independently.

We also studied the genome of a species of Rhizobiales bacteria found in ants. We discuss its metabolic capabilities and suggest scenarios for how it may affect the ants’ lifestyle. This genome contained a region with homology to the Bartonella gene transfer agent (GTA), which is a domesticated bacteriophage used to transfer bacterial DNA between cells. We propose that its unique behaviour as a specialist GTA, preferentially transferring host-interaction factors, originated from a generalist GTA that transferred random segments of chromosomal DNA.

These bioinformatic analyses of previously uncharacterized bacterial lineages have increased our understanding of their physiology and evolution and provided answers to old and new questions in fundamental microbiology.